A method is described for the determination of potassium in seawater and marine sediment porewater. After precipitation of the halogen ions with AgNO3, potassium is precipitated with sodium tetraphenylborate. The excess tetraphenylborate is then determined by AgNO3titration, using a silver electrode for end-point detection. Application of this method to seawater samples as small as 1 cm3, gave accuracy and precision of better than 1%.